Apparatus and method for communicating data over a communication channel

ABSTRACT

For some applications such as high-speed communication over short-reach links, the complexity and associated high latency provided by existing modulators may be unsuitable. According to an aspect, the present disclosure provides a modulator that can reduce latency for applications such as 40 G/100 G communication over copper cables or SMF. The modulator has a symbol mapper for mapping a bit stream into symbols, and a multi-level encoder including an inner encoder and an outer encoder for encoding only a portion of the bit stream. In some implementations, the multi-level encoder is configured such that an information block size of the inner encoder is small and matches a field size of the outer encoder. Therefore, components that would be used to accommodate larger block sizes can be omitted. The effect is that complexity and latency can be reduced. According to another aspect, the present disclosure provides a demodulator that is complementary to the modulator.

RELATED APPLICATION

This application is a continuation of U.S. patent application Ser. No.15/206,011 filed Jul. 8, 2016, which is a continuation of U.S. patentapplication Ser. No. 14/744,015 filed on Jun. 18, 2015, now U.S. Pat.No. 9,413,493 issued Aug. 9, 2016, which in turn is a continuation ofU.S. patent application Ser. No. 14/180,315 filed on Feb. 13, 2014, nowU.S. Pat. No. 9,083,492 issued Jul. 14, 2015, which claims priority fromU.S. Provisional Patent Application No. 61/765,050 filed on Feb. 15,2013. The aforementioned applications are incorporated by reference intheir entirety.

FIELD OF THE DISCLOSURE

The disclosure relates to communication systems, and more particularlyto communicating data over a communication channel.

BACKGROUND

A transmitter can transmit a signal to a receiver over a communicationchannel. The signal can encode various data. If the communicationchannel is not perfect, then the signal received by the receiver may notbe identical to the signal transited by the transmitter. For instance,the communication channel may have introduced noise into the signal. Ifthe signal received by the receiver is distorted enough by thecommunication channel, then the data may not be recoverable by thereceiver.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the disclosure will now be described with reference tothe attached drawings in which:

FIG. 1 is a block diagram of a binary encoder;

FIG. 2 is a block diagram of a convolutional encoder;

FIG. 3 is a block diagram of a TCM (trellis-coded modulator) featuringan encoder that encodes only a portion of a bit stream;

FIG. 4 is a constellation diagram representing a set of example symbolsfor the trellis-coded modulator shown in FIG. 3;

FIG. 5 is a block diagram of the encoder shown in FIG. 3;

FIG. 6 is a block diagram of a multi-level modulator in accordance withan embodiment of the disclosure;

FIG. 7 is a constellation diagram representing a set of example symbolsfor the multi-level modulator shown in FIG. 6;

FIG. 8 is a graph depicting SNR (Signal to Noise Ratio) vs. Latency forthe multi-level modulator shown in FIG. 6; and

FIG. 9 is a block diagram of another multi-level modulator in accordancewith an embodiment of the disclosure.

DETAILED DESCRIPTION OF EMBODIMENTS

It should be understood at the outset that although illustrativeimplementations of one or more embodiments of the present disclosure areprovided below, the disclosed systems and/or methods may be implementedusing any number of techniques, whether currently known or in existence.The disclosure should in no way be limited to the illustrativeimplementations, drawings, and techniques illustrated below, includingthe exemplary designs and implementations illustrated and describedherein, but may be modified within the scope of the appended claimsalong with their full scope of equivalents.

For some applications such as high-speed communication over short-reachlinks, the complexity and associated high latency provided by existingmodulators may be unsuitable. According to an aspect, the presentdisclosure provides a modulator that can reduce latency for applicationssuch as 40 G/100 G communication over copper cables or SMF (single-modefiber). The modulator has a symbol mapper for mapping a bit stream intosymbols, and a multi-level encoder including an inner encoder and anouter encoder for encoding only a portion of the bit stream. In someimplementations, the multi-level encoder is configured such that aninformation block size of the inner encoder is small and matches a fieldsize of the outer encoder. Therefore, components that would be used toaccommodate larger block sizes can be omitted. The effect is thatcomplexity and latency can be reduced. According to another aspect, thepresent disclosure provides a demodulator that is complementary to themodulator.

Other aspects and features of the present disclosure will becomeapparent, to those ordinarily skilled in the art, upon review of thefollowing description of the specific embodiments of the invention.

Introduction

If a signal received by a receiver is distorted enough by acommunication channel, then data sent with the signal may not berecoverable by the receiver. FEC (forward error correction) or channelcoding is a technique that involves encoding a signal in a redundantmanner so that data sent with the signal may be recoverable even ifthere are errors in transmission. Such techniques may be employed whenit is difficult or impractical to support re-transmission of datafollowing an error. Example encoders are described below with referenceto FIGS. 1 and 2.

Referring first to FIG. 1, shown is a block diagram of a binary encoder100. The binary encoder 100 is capable of encoding an initial bit streamk into an encoded bit stream n, where n is an integer multiple of k. Forexample, if the integer multiple is three (i.e. rate=1/3) and theinitial bit stream is “01”, then the encoded bit stream would be“010101”. Note that if any one bit of the encoded bit stream is changedduring transmission (e.g. due to a noisy communication channel), then itmay still be possible to recover the initial bit stream because therewould be two recovered copies of the initial bit stream against one badcopy of the initial bit stream.

The binary encoder 100 allows for adequate FEC capabilities. However,the binary encoder 100 is not very efficient at utilizing bandwidth of acommunication channel. In the foregoing example with the rate=1/3, itcan be seen that two-thirds of the bits transmitted over thecommunication channel are redundant. Reducing the rate to 1/2 reducesthe amount of redundant bits, but if any one bit of the encoded bitstream is changed during transmission, then it may not be possible torecover the initial bit stream because there would be only one recoveredcopy of the initial bit stream against one bad copy of the initial bitstream and the receiver may not know which is the recovered copy.

The binary encoder 100 is rather rudimentary because the encoded bitstream n is merely made up of exact copies of the initial bit stream k.If the encoded bit stream n is transmitted over a communication channelthat tends to cause an error for a certain bit pattern, then theredundancy provided by the encoded bit stream n may be inadequate (i.e.the same error may affect all copies of the initial bit stream k). Insuch scenarios, increasing redundancy provided by the binary encoder 100may not help.

Referring now to FIG. 2, shown is a block diagram of a convolutionalencoder 200, which may provide improved redundancy compared to thebinary encoder 100 shown in FIG. 1. The convolutional encoder 200includes various delay elements 211,212,213 and various summationelements 221,222. The number of delay elements 211,212,213 and theconnections with the summation elements 221,222 determine how outputs V₁and V₂ are derived from an input u. In the illustrated example, theoutputs V₁ and V₂ are provided by equations 1 and 2 below.V ₁ =u[n]+u[n−2]+u[n−3]  [1]V ₂ =u[n−1]+u[n−2]  [2]Of course different delay elements 211,212,213 and different connectionswith the summation elements 221,222 would provide different outputs V₁and V₂.

In the illustrated example, the convolutional encoder 200 has a rate of1/2 because there are two outputs V₁ and V₂ for one input u. Other ratesare possible for example by increasing the number of outputs. Incontrast with the binary encoder 100 shown in FIG. 1, the outputs V₁ andV₂ may be quite different from one another. Thus, if a communicationchannel tends to cause an error for a certain bit pattern found in oneoutput, this may not affect the other output if it does not have thatbit pattern. In this regard, the outputs V₁ and V₂ may be specificallydesigned so that they are quite different from one another. Note that ifthe outputs V₁ and V₂ are designed to be identical to one another, thenthe convolutional encoder 200 may not provide improved redundancy overthe binary encoder 100 shown in FIG. 1.

The binary encoder 100 shown in FIG. 1 and the convolutional encoder 200shown in FIG. 2 may be used to encode all bits of a bit stream. However,the amount of redundancy involved in encoding all bits of a bit streamcan be expensive from a bandwidth perspective. For example, whenencoding all bits with a rate=1/3, two-thirds of the bits transmittedover a communication channel are redundant and thus represent aconsiderable amount of overhead that consumes bandwidth. Greatertransmission rates may be achievable if the amount of overhead can bereduced. This can be accomplished by selectively encoding a portion of abit stream.

Trellis-Coded Modulator

Referring now to FIG. 3, shown is a block diagram of trellis-codedmodulator 300 featuring an encoder 320 that encodes only a portion of abit stream. In particular, the encoder 320 encodes k bits while aremaining m−k bits may not be encoded. The choice of which bits toencode is implementation specific, but generally involves encoding bitsfor which errors may make it difficult or impossible to recover data. Inthe illustrated example, the encoded bits k+d are used to select asubset of symbols while the remaining m−k bits are used to select asymbol from the subset of symbols. There are 2^(m+d) PAM (PulseAmplitude Modulation) symbols in the constellation. A signal mapper 310maps information bits and convolutionally encoded bits to aconstellation point.

Referring now to FIG. 4, shown is a constellation diagram 400representing a set of example symbols for the trellis-coded modulatorshown in FIG. 3. Note that the constellation diagram 400 and its examplesymbols are provided merely for explanatory purposes. The constellationdiagram 400 includes four classes of symbols represented by four shapes:circle, square, diamond, and triangle. The encoded bits k+d can be usedto select one class of symbols, for example the class of symbolsrepresented by the diamond. Note that all symbols of a class areseparated from one another in Euclidean space. Thus, once a class ofsymbols is selected, the remaining m−k bits can be used to select thesymbol of that class. When and if there is any error in transmission forthe remaining m−k bits, it may still be possible to decipher whichsymbol of that class was transmitted, as other symbols of that class arefar away in a Euclidean sense.

The encoder 320 of the trellis-coded modulator 300 shown in FIG. 3 mightfor example be a convolutional encoder that encodes k bits into k+1 bitsthereby achieving a rate=k/(k+1). Such rate can enable efficientdecoding by a receiver using a soft-decision Viterbi decoder. However,other rates are possible and are implementation-specific. In otherimplementations, the encoder 320 includes multiple levels formulti-level (and/or multi-phase) modulation. Thus, the trellis-codedmodulator 300 can combine binary error-correcting codes with multi-level(and/or multi-phase) modulation. This can help to reliably communicateat high spectral-efficiencies. Example details of the encoder 320including multiple levels are provided below. Note that for suchimplementations, the encoder 320 may have a different rate than k/(k+1).

Referring now to FIG. 5, shown is a block diagram of the encoder 320shown in FIG. 3. The encoder 320 has two levels including a block code321 and a convolutional code 323. The encoder 320 also has aninterleaver 322, which will be described later. In operation, the blockcode 321 performs coding on an input sequence of k bits. The block code321 might for example generate a 200-bit sequence from a 100-bitsequence by creating redundancy in a similar manner as in the binaryencoder 100 shown in FIG. 1. The convolutional code 323 operates tofurther code such redundancy. The combination of the block code 321 andthe convolutional code 323 can help to reliably communicate symbols athigh spectral-efficiencies.

When each symbol transmitted by the trellis-coded modulator 300 isreceived at a receiver (not shown), the symbol may be demapped anddecoded in a manner that is complementary to the trellis-coded modulator300. In this regard, the receiver may include a demapper for demappingthe symbols, a deconvolution block for decoding the bits that have beenencoded using the convolutional code 323, and another decoder block fordecoding the bits that have been encoded using the block code 321. Themanner in which this is accomplished is implementation-specific.

If there is an error during transmission, then the deconvolution blockmay perpetuate such error from one bit to other neighbouring bits. Thisis a by-product of the nature of convolution and deconvolution. If manyerrors exist for one outer codeword (i.e. codeword from an outerencoder), then it may not be possible to recover that outer codeword. Inorder to address this issue, the trellis-coded modulator 300 and itscorresponding receiver are designed to spread out the errors over manyouter codewords so that ideally no single outer codeword contains moreerrors than can be corrected. This is made possible by the interleaver322, which loads many outer codewords into memory, interleaves the outercodewords, and outputs interleaved outer codewords.

Example operation of the interleaver 322 will be depicted with exampleoriginal outer codewords show in Table 1 and interleaved outer codewordsshown in Table 2.

TABLE 1 Example Input to Interleaver 322 Original Outer Codeword Bits 1u₁, u₂, u₃, . . . , u₁₀₀₀ 2 u₁, u₂, u₃, . . . , u₁₀₀₀ 3 u₁, u₂, u₃, . .. , u₁₀₀₀ . . . . . . 1000   u₁, u₂, u₃, . . . , u₁₀₀₀The interleaver 322 creates an interleaved codeword using the first bitof each original codeword, and repeats for the remaining bits.

TABLE 2 Example Output from Interleaver 322 Interleaved Outer CodewordBits 1 u₁, u₁, u₁, . . . , u₁ 2 u₂, u₂, u₂, . . . , u₂ 3 u₃, u₃, u₃, . .. , u₃ . . . . . . 1000   u₁₀₀₀, u₁₀₀₀, u₁₀₀₀, . . . , u₁₀₀₀

When an error in transmission affects an interleaved outer codeword, theresult is that the error may affect multiple original outer codewordsbeing recovered. However, since the errors are spread out over manyoriginal outer codewords being recovered, it may still be possible torecover all original outer codewords. Therefore, the interleaver 322plays a useful role in increasing the robustness of the trellis-codedmodulator 300.

However, since the interleaver 322 loads many original outer codewordsinto memory before it can generate the interleaved outer codewords, theinterleaver 322 introduces significant latency. Also, since the receiverwould implement a de-interleaver that loads many interleaved outercodewords before it can try to recover the original outer codewords, thede-interleaver also introduces significant latency. Such latencies maybe tolerable for some applications, but for some applications they arenot. For instance, for high-speed 40 G/100 G communication over SMF, thesignificant latencies introduced by the interleaver 322 and thecomplementary de-interleaver are generally considered to be poor orimpractical. Simply removing the interleaver 322 and the complementaryde-interleaver may reduce the robustness of the trellis-coded modulator300 and the receiver and therefore may not be a practical solution.

Multi-Level Modulator

Referring now to FIG. 6, shown is a block diagram of a multi-levelmodulator 600, which can allow for lower latencies compared to thetrellis-coded modulator 300 shown in FIG. 5. The multi-level modulator600 has a symbol mapper 610 for mapping a three-bit sequence (b₂, b₁,b₀) into 8-PAM symbols. A constellation diagram representing a set ofexample symbols for the multi-level modulator 600 is shown in FIG. 7.Note that the LSB (Least Significant Bit) of the symbols are gray coded(i.e. they alternate 0, 1, 0, 1, etc.). Given knowledge of b₀, theinduced sub-constellation is a uniformly-spaced 4-PAM (this is theset-partitioning element). This means that the LSB can be used to selecta class of symbols (i.e. class having LSB=0 or class having LSB=1) witheach symbol of that class being separated from one another (i.e. notneighbouring).

Therefore, with reference back to FIG. 6, the LSB sequence is encoded bymultiple levels including an outer encoder 621 and an inner encoder 622so that the LSB sequence may be recoverable by a receiver (not shown) inthe event of transmission errors. The outer encoder 621 might forexample be an RS (Reed-Solomon) encoder 621 as shown, or some otherencoder such as a BCH (Bose Ray-Chaudhuri Hocquenghem) encoder or otheralgebraic code. The RS encoder 621 encodes k_(RS)·k_(B) bits inton_(RS)·k_(B) bits, where n_(RS)>k_(RS). The inner encoder 622 might forexample be a block code as apposed to a convolutional code. The innerencoder 622 encodes n_(RS)·k_(B) bits into n_(RS)·n_(B) bits, wheren_(B)>k_(B). The multi-level modulator 600 uses the constellation havingmixed gray/set-partitioned labelling shown in FIG. 7 such that themulti-level modulator 600 can be considered to apply BICM(bit-interleaved coded-modulation) on the gray-labelled bits and MLC(multi-level-coding) on the set-partitioned bits. The output of theinner encoder 622 is applied to the gray-labelled bits, while theset-partition-labelled bits are uncoded or protected by a high-ratecode.

In accordance with an embodiment of the disclosure, for the innerencoder 622, short codes with efficient trellis representations (e.g.single-parity-check, Hamming, and extended-Hamming codes) are used. Anadvantage of using short codes (instead of using a convolutional code asin the TCM 300 depicted by FIGS. 3 and 5) arises from their short andterminated trellises, which can reduce the complexity of hardwareimplementations of encoding at the transmitter and decoding at thereceiver (not shown). In particular, if the information block size k_(B)of the inner encoder 622 is chosen to match the field size m of theouter encoder 621, then an interleaver between the inner encoder 622 andthe outer encoder 621 can be omitted because any errors would beconfined to symbol boundaries.

The inner encoder 622 is positioned between the outer encoder 621 andthe symbol mapper 610. Concatenation of multiple codes is a technique toconstruct a powerful code from simple components, while maintaining arelatively simple decoder. Furthermore, it allows codes withcomplementary properties to be combined into a single code. For example,the inner code is typically a code with an efficient soft-decisiondecoder, whereas the outer code is a code with an efficienthard-decision decoder. In the language of the previous paragraph, theinner code is said to have an efficient trellis representation, whichimplies an efficient soft-decision decoder.

In comparison with the trellis-coded modulator 300 depicted by FIGS. 3and 5, the multi-level modulator 600 shown in FIG. 6 replaces theencoder 320 including the convolutional code 323 and the interleaver 322with the inner encoder 622 (e.g. block code) concatenated with the outerencoder 621. Note that interleaving is not required for the multi-levelmodulator 600 because any errors would be confined to symbol boundaries.However, in alternative implementations, the information block sizek_(B) of the inner encoder 622 is some small multiple (e.g. exactly two)of the field size m and thus there could be interleaving over thismultiple of codewords to deal with the fact that errors would no longerbe confined to symbol boundaries. However, such alternativeimplementations might provide poorer performance than if the informationblock size k_(B) of the inner encoder 622 is chosen to match the fieldsize m of the outer encoder 621.

Various examples described herein refer to an “LSB sequence”, which is aterm used to describe a sequence of bits where each bit is an LSB (i.e.b₀) of a set of bits (e.g. (b₂, b₁, b₀) for 8-PAM) that are mapped to asymbol. Thus, if an LSB sequence has 100 bits for example, the LSBsequence may be used to map 100 symbols. Typically a single LSB is usedto select a class of symbols. However, in another embodiment, more thanone bit could be used for this purpose. Thus, an “LSB sequence” couldalternatively be a sequence of bits where each bit is alower-significance subset (e.g. b₀ or b₁,) of a set of bits (e.g. (b₂,b₁, b₀) for 8-PAM) that are mapped to a symbol.

According to the multi-level modulator 600 shown in FIG. 6, the outerencoder 621 and the inner encoder 622 encode only a portion of the bitstream, namely an LSB sequence. However, it is to be understood thatother implementations are possible and are within the scope of thisdisclosure. For instance, in another implementation, the outer encoder621 and the inner encoder 622 encode some other portion of the bitstream. Such other portion may include a non LSB sequence and/or an MSB(Most Significant Bit) sequence. In an alternative implementation, theouter encoder 621 and the inner encoder 622 encode the entire bit stream(i.e. not just a portion of the bit stream).

Note that defining the constellation with 8-PAM symbols isimplementation-specific. In other implementations, 4-PAM symbols areused. In other implementations, 16-PAM symbols are used. More generally,any suitable constellation with 2^(N)-PAM symbols may be employed. For2^(N)-PAM, each of the n_(RS)·n_(B) bits of the LSB sequence maps to theleast significant bit (i.e. b₀) in the label of a 2^(N)-PAM symbol, withthe overall label being (b₀, b₁, . . . , b_(N-1)). A correspondingstream of (N−1)·n_(RS)·n_(B) most significant bits, mapped to (b₁, b₂, .. . , b_(N-1)), are optionally encoded by the RS encoder 630. Note thatthe size of the constellation may be unrelated to the length of theblock code. For instance, according to some implementations, only theLSB of the constellation (assuming PAM modulation) is encoded by aninner/outer encoder, regardless of the number of points in theconstellation.

The non-LSB sequences may be encoded by a separate encoder 630, but thisis completely optional because it may be easy for the receiver todecipher which symbol has been transmitted even when there are errorsduring transmission. This is because all symbols of a class selected bythe LSB are separated from one another. The separate encoder 630 mightfor example be an RS encoder 630 as depicted, or some other encoder suchas BCH encoder.

The multi-level modulator 600 is shown with two levels of encoding (i.e.the inner encoder 622 and the outer encoder 621). However, inalternative implementations, there are three or more levels of encoding.For example, a two-dimensional outer code could be used, where“codewords” are 2-D arrays of m-bit symbols, each row and column of thearray being a valid codeword of an RS code. The two-dimensional codesmay be referred to as “Product Codes”, and could be considered to be atwo-level outer encoding. However, such alternative implementations withthree or more levels of encoding might provide higher latency than twolevels of encoding. More generally, the multi-level modulator 600 has atleast two levels of encoding.

The multi-level modulator 600 can provide advantages with respect tolatency and hardware complexity, which are two of the main challenges indesigning FEC schemes for high-speed short-reach links. For instance,since the multi-level modulator 600 operates to confine errors to symbolboundaries with short codes, interleaving can be omitted therebyreducing hardware complexity and allowing lower latencies.

Note that the latencies can be designed. For example, by fixing thechoice of the inner block code and the overall spectral efficiency, arange of latencies can be obtained by varying the (n_(RS),k_(RS)=n_(RS)−2t_(RS)) of the RS encoder 621, where t_(RS) is a maximumnumber of m-bit RS symbols that can have error. In each case, the fieldsize is m=k_(B), and thus a single RS decoder can be designed (for themaximum t_(RS)) that can be used for the entire range of latencies. Forthe specific case of a target spectral efficiency of 2.60 bits/symboland a (n_(B)=16, k_(B)=11) extended-Hamming inner code, Table 3 givesthe parameters of the RS encoder 621 for a range of latencies. Note thatin each case, the field size of the RS encoder 621 is m=11.

TABLE 3 Example Parameters for RS Encoder 621 n_(RS) k_(RS) t_(RS) 157137 10 236 206 15 314 274 20 393 343 25 471 411 30 550 380 35 629 549 40

Referring now to FIG. 8, shown is a graph 800 depicting SNR (Signal toNoise Ratio) vs. latency for the multi-level modulator 600 with theforegoing parameters on an additive white Gaussian noise channel. Notethat lower latencies are achieved with small code blocks (i.e. smalln_(RS) and k_(RS)), but at the same time this reduces the maximum numberof m-bit RS symbols that can have error (i.e. t_(RS)). The graph 800suggests that higher SNR can allow for lower latencies. Higher SNR maybe achieved for example by increasing power of transmission, which mayreduce power efficiency. Therefore, the precise configuration of themulti-level modulator 600 is implementation-specific and may depend onvarious tradeoffs (e.g. latency vs. SNR).

It is to be understood that the field size of the outer encoder 621being m=11 is very specific for exemplary purposes only. Other fieldsizes are possible and are within the scope of this disclosure. Forexample, in other implementations, the field size of the outer encoder621 is m=9. Such implementations may for example use 5 parity bits (e.g.14 bits total), or some other number of parity bits. The informationblock size of the inner encoder 622 may be k_(B)=9 in the event that itis chosen to match the field size m of the outer encoder 621. In someimplementations, the field size of the outer encoder 621 is M≥9, and theinformation block size of the inner encoder 622 is the same in the eventthat it is chosen to match the field size m of the outer encoder 621.Other implementations are possible.

The precise configuration of the multi-level modulator 600 wouldnormally be static based on a hardware configuration of the multi-levelmodulator 600. However, in alternative implementations, the preciseconfiguration of the multi-level modulator 600 can be dynamicallychanged. Dynamically changing the multi-level modulator 600 could forexample allow the multi-level modulator 600 to accommodate for differentlatencies under different operating environments, etc.

Another Multi-Level Modulator

Referring now to FIG. 9, shown is a block diagram of another multi-levelmodulator 900. The multi-level modulator 900 has a symbol mapper 910 formapping an N-bit (b₀, b₁, . . . , b_(N-1)) stream into 2^(N)-PAMsymbols. The N-bit stream includes an LSB sequence of n_(RS)·n_(B) bitsencoded by a serial concatenation of an outer encoder 921 and an innerencoder 922 as similarly described for the multi-level modulator 600shown in FIG. 6. In some implementations, the field size of the outercode matches the information block size of the inner code, as similarlydescribed with reference to FIG. 6.

However, in contrast with the multi-level modulator 600 shown in FIG. 6,“N” is an integer that may vary from symbol to symbol. For example, thefirst bit in the LSB encoded stream might map to the LSB in the label ofan 8-PAM symbol, the second bit in the LSB encoded stream might map tothe LSB in the label of a 4-PAM symbol, the third bit in the LSB encodedstream might map to the LSB in the label of a 2-PAM symbol, etc.According to this example, for each symbol mapped by the symbol mapper910, N can be 1, 2 or 3. Note that there may be sequential symbols ofthe same constellation despite “N” being a variable. Thus, “N” may bethe same value for some sequential symbols, but change for othersequential symbols.

For each 2^(N)-PAM symbol in the stream, the non-LSB bits are obtainedfrom a common stream of p·n_(RS)·n_(B) bits, which are optionallyencoded by an RS encoder 930. Note that “p” is an average number of bitsper LSB and is not necessarily an integer. For example, if half of thesymbols are 4-PAM and the other half are 8-PAM, then p=(1+2)/2=1.5. Thenumber of bits per LSB varies with N. In some implementations, thechoice of N depends on a “bit-loading” algorithm, which selects N basedon one or more criteria that may depend on modulation used.

The “variable” 2^(N)-PAM modulation described with reference to FIG. 9may be utilized with DMT modulation, which is also known as OFDM(Orthogonal Frequency Division Multiplexing) modulation. In DMT,inherent frequency division multiplexing dictates that different symbolsare transmitted over different frequency bands. Note that each frequencyband has its own signal-to-noise ratio, and higher signal-to-noiseratios imply larger capacity for communication. The task of thebit-loading algorithm is to select N as a function of thesignal-to-noise ratio of the corresponding frequency band.

More specifically, in DMT, a frequency band is divided into Msub-channels. To achieve a desired rate of communication, L bits ofinformation are transmitted per DMT symbol. A DMT symbol is effectivelyM PAM-N sub symbols, where N_(i), i in [1,M], is chosen by thebit-loading algorithm. In order to transmit L bits of information, thebit-loading algorithm is constrained to find the N_(i) that result in atarget value of “p” (the average number of bits per LSB), where thisvalue is a function of the rates of the codes applied to the LSB andnon-LSB sequences.

Note that a DMT transmitter does not actually sequentially transmitvarying 2^(N)-PAM symbols sequentially in time over a communicationchannel. Rather, it actually takes an IDFT (Inverse Discrete FourierTransform) of the sequence of 2^(N)-PAM symbols, and then transmits thisresulting waveform over the communication channel. In doing so, it isdoing frequency division multiplexing of many sub-channels, eachsub-channel communicating a 2^(N)-PAM modulation.

It is to be understood that the mapping of encoded bits to a “variable”2^(N)-PAM modulation is not specific to DMT, although DMT is anapplication where this may be useful. Note that the encoding of the LSBsequence may occur regardless of modulation utilised (e.g. 2^(N)-PAM orDMT). However, the encoding of the non-LSB sequence may depend on themodulation utilised as described above. Also note that a PAMconstellation may be used regardless of modulation scheme (e.g. PAM andDMT modulation).

Multi-Level Demodulator

Although not depicted, the present disclosure also includes amulti-level demodulator that is complementary to the multi-levelmodulator 600 shown in FIG. 6 and/or the multi-level modulator 900 shownin FIG. 9. The multi-level demodulator may include a demapper fordemapping symbols, a first decoder block for decoding the bits that havebeen encoded using an inner encoder, and a second decoder block fordecoding the bits that have been encoded using an outer encoder. Themanner in which this is accomplished is implementation-specific.

Additional aspects are defined by the following clauses:

-   Clause 1. A demodulator comprising:

a symbol demapper for demapping symbols into a bit stream; and

a multi-level decoder comprising an inner decoder and an outer decoderfor decoding only a portion of the bit stream such that an informationblock size of the inner decoder matches a field size of the outerdecoder.

-   Clause 2. The demodulator of Clause 1, wherein the portion of the    bit stream comprises an LSB (Least Significant Bit) sequence.-   Clause 3. The demodulator of Clause 1 or Clause 2, wherein:

the bit stream further comprises a non-LSB sequence having (N−1)-timesas many bits as the LSB sequence; and

the symbol demapper is configured for demapping the symbols from aconstellation of 2^(N)-PAM symbols into the bit stream, where N is aninteger and is constant from symbol to symbol.

-   Clause 4. The demodulator of Clause 1 or Clause 2, wherein:

the bit stream further comprises a non-LSB sequence having p-times asmany bits as the LSB sequence, where p is an average number of bits perLSB; and

the symbol demapper is configured for demapping the symbols from aconstellation of 2^(N)-PAM symbols into the bit stream, where N is aninteger and is variable from symbol to symbol.

-   Clause 5. The demodulator of any one of Clause 1 to Clause 3,    wherein:

the inner decoder is a block decoder; and

the outer decoder is an RS (Reed-Solomon) decoder.

-   Clause 6. A method for demodulating symbols into a bit stream,    comprising:

demapping symbols into the bit stream; and

a multi-level decoder comprising an inner decoder and an outer decoderdecoding only a portion of the bit stream such that an information blocksize of the inner decoder matches a field size of the outer decoder.

-   Clause 7. A demodulator comprising:

a symbol demapper for demapping symbols into a bit stream, the bitstream comprising an LSB (Least Significant Bit) sequence; and

a multi-level decoder comprising an inner decoder and an outer decoderfor decoding only the LSB sequence of the bit stream.

-   Clause 8. The demodulator of Clause 7 wherein the inner encoder and    the outer encoder are configured such that an information block size    of the inner encoder matches a field size of the outer encoder.-   Clause 9. A method for demodulating symbols into a bit stream,    comprising:

demapping symbols into the bit stream, the bit stream comprising an LSB(Least Significant Bit) sequence; and

a multi-level decoder comprising an inner decoder and an outer decoderdecoding only the LSB sequence of the bit stream.

Numerous modifications and variations of the present disclosure arepossible in light of the above teachings. It is therefore to beunderstood that within the scope of the appended claims, the disclosuremay be practised otherwise than as specifically described herein.

The invention claimed is:
 1. A modulator comprising: a symbol mapper formapping a bit stream into symbols; and a multi-level encoder comprisingan inner encoder and an outer encoder for encoding only a portion of thebit stream such that an information block size of the inner encoder is amultiple of a field size of the outer encoder; wherein a portion of thebit stream comprises a least significant bit (LSB) sequence and anon-LSB sequence having p-times as many bits, p being an average numberof bits per LSB; and the symbol mapper being configured for mapping thebit stream into the symbols from a constellation of 2^(N) symbols, whereN is an integer and is variable from symbol to symbol.
 2. The modulatorof claim 1, wherein the multiple is a small multiple.
 3. The modulatorof claim 2, wherein the small multiple is exactly two.
 4. The modulatorof claim 1, wherein the portion of the bit stream comprises an LSB(Least Significant Bit) sequence.
 5. The modulator of claim 4, whereinthe symbol mapper performs the mapping without a non-LSB sequence of thebit stream being encoded.
 6. The modulator of claim 4, wherein: the bitstream further comprises a non-LSB sequence having (N−1)-times as manybits as the LSB sequence; and the symbol mapper is configured formapping the bit stream into symbols from a constellation of 2^(N)-PAMsymbols, where N is an integer and is constant from symbol to symbol. 7.The modulator of claim 6 wherein a bit from each of the symbols is graycoded.
 8. The modulator of claim 6 wherein the constellation has mixedgray/set-partitioned labelling.
 9. The modulator of claim 4, wherein:the bit stream further comprises a non-LSB sequence having p-times asmany bits as the LSB sequence, where p is an average number of bits perLSB; and the symbol mapper is configured for mapping the bit stream intosymbols from a constellation of 2^(N) symbols, where N is an integer andis variable from symbol to symbol.
 10. The modulator of claim 4, furthercomprising: a second encoder for encoding a non-LSB sequence of the bitstream.
 11. The modulator of claim 10, wherein the second encoder is anRS (Reed-Solomon) encoder.
 12. The modulator of claim 1, wherein thefield size of the outer encoder is at least
 9. 13. The modulator ofclaim 1, wherein: the inner encoder is a block encoder; and the outerencoder is an RS (Reed-Solomon) encoder.
 14. The modulator of claim 1,wherein: the inner encoder is a Hamming or Extended Hamming encoder; andthe outer encoder is an RS (Reed-Solomon) encoder.
 15. The modulator ofclaim 1, wherein: the inner encoder is a single-parity-check encoder;and the outer encoder is an RS (Reed-Solomon) encoder.
 16. The modulatorof claim 1, wherein: the outer encoder is a Bose Ray-ChaudhuriHocquenghem (BCH) encoder.
 17. A method for modulating a bit stream intosymbols, comprising: encoding only a portion of a bit stream comprisingan LSB (Least Significant Bit) sequence and a non-LSB sequence by amulti-level encoder comprising an inner encoder and an outer encoder,such that an information block size of the inner encoder is a multipleof a field size of the outer encoder, the non-LSB sequence havingp-times as many bits as the LSB sequence, where p being an averagenumber of bits per LSB; and mapping the bit stream into the symbols froma constellation of 2^(N) symbols, where N is an integer and is variablefrom symbol to symbol.
 18. The method of claim 17, further comprising:encoding, by a second encoder, a non-LSB sequence of the bit stream. 19.The method of claim 17, wherein: the bit stream further comprises anon-LSB sequence having (N−1)-times as many bits as the LSB sequence;and the mapping comprises mapping the bit stream into the symbols from aconstellation of 2^(N)-PAM symbols, where N is an integer and isconstant from symbol to symbol.